Pipe installations are subject to fatigue and potential damage which greatly benefit from permanently installed sensors for monitoring the status of these structures. Such monitoring aids in mitigating the risks associated with a possible loss of integrity. When monitoring such structures, it is often desirable to install the monitoring instrumentation intimately with the structures. One such monitoring objective is to measure the structural strain developed by the monitored system under a load.
Sensor technologies which could be used for such monitoring include: Linear Variable Differential Transformer (LVDT), Wheatstone bridges, and Fiber Optic with Bragg Grating. Generally, the strain sensors are mounted on a frame or on collars that constitute an interface between the structural element being monitored and the sensor itself. These interfaces allow for recovering and reinstalling the sensors onto the structural element for maintenance or repair. The sensor could also be directly bonded to the assets, which often makes it difficult or impossible to maintain or repair the sensor in case of sensor failure.
One example of such pipe installations relates to subsea applications wherein the structures are generally exposed to severe environmental conditions. Subsea hydrocarbon production systems using sea surface facilities of any sort require petroleum fluids to flow from the seabed to the surface through pipes called risers. The sea surface rises and falls with waves and tides, and the facilities are moved vertically, laterally and rotationally by various forces. The risers can either be steel pipes relying on their intrinsic flexibility or a range of flexible composite materials that are designed to resist the internal conveyance of fluids and the external forces imposed by all foreseen conditions. In another example, single line offset risers (SLHR) consist of a column of rigid pipe firmly attached to a foundation on the seabed and supported near the mean water level by a buoyancy can or tank. It is vital that these risers do not leak petroleum fluids to the environment, and do not suffer mechanical failures, which would require production to be stopped, causing severe loss of revenue. It might thus be beneficial to monitor integrity of such installations to mitigate risks associated with possible failure.
A typical field of application for subsea is a subsea oil and gas field architecture that integrates a pipeline network to transport the production fluid from the wellhead to the surface facilities. As part of this pipeline network the riser pipe structure is provided close to the surface process facilities to lift the fluid from the seabed to the surface. In deep and ultra deep water examples, operators have often adopted the hybrid free-standing riser concept which comprises: a seabed riser anchor base; a vertical single or bundled riser pipe(s) anchored to the seabed; a buoyancy tank providing an uplift tension to vertical riser pipe(s); a flexible pipe connecting the top of the vertical riser to the surface process facilities (FPSO); and a flexible joint connecting the buoyancy tank to the vertical riser. Accidental flooding of the buoyancy tank could create a potential hazard to the riser system and expose the field to catastrophic failure if a sufficient uplift tension is not applied to the vertical pipe system. In some applications, the buoyancy tank is made up of several independent compartments to limit the amount of water that could accidentally fill the tank. In order to further mitigate risks, subsea operators often request to install instrumentation to monitor possible accidental flooding of the buoyancy tank.
Generally, operators request that the tension generated by the buoyancy tank be monitored by means of an integrity monitoring system equipped with gages able to measure the pipe strain as shown. Such a system, well suited for detecting a sudden event, is more limited in the case of a slow water intrusion inside the tank, for example, due to corrosion. Readings collected from the tension collar may drift and the instrument cannot be recalibrated subsea. As a result, it is difficult to differentiate real water ingress from the data drift. Further, in some cases it is desirable to provide a secondary and independent monitoring system for redundancy and increased security.